The invention relates to modified cellulose products comprising cellulose ether, and a ligand. The invention also relates to a method for the preparation of modified cellulose products, and to drilling fluids containing such products.
When drilling for oil-, gas-, water-, or other subsoil targets, the drilling process is most commonly performed with a revolving pipe equipped with a device called a bit. This system depends upon the rotation of the column of drill pipe to the bottom of which is attached a multi prolonged drilling bit. The drill bit cuts into the earth, causing to accumulate cuttings as drilling continues. As a result, a fluid must be used to carry these cuttings to the surface for removal, thus allowing the bit to continue functioning and the bottom of the hole to be kept clean and free of cuttings at all times.
When the drilling process is on going, the drilling fluid has to protect the borehole wall from deformation and/or deterioration resulting from reactions of the sediments or formation with the liquid and other containing elements of the said fluid. These chemical and/or physical reactions are resulting from the osmotic and/or crystalline effects. One specific problem in this respect is so called shale hydration.
Shales are a group of mainly clay minerals reacting in various forms with water and salts that might be present during the drilling process or in the underground. This reaction might hamper the drilling process, and in the worst case the process must be totally stopped.
Shales are minerals belonging to the smectic group. The most well known is montmorillonite. Normally this is the most active component in argillaceous formations causing hydrodynamic volume increase when exposed to aqueous environments, called swelling, and problems when drilling.
Illites, kaolinite and chlorite are other members of the active clay group being responsible for well problems based on reaction of ions and water with these clays. These clays can be highly dispersive. In nature these clays are found in a mixed composition with more or less severe potential problems when drilling through.
Shale stabilisation is believed to at least partially be dependent upon inhibition of swelling and dispersion process of the shale by the fluid.
Shales and shale reactions are extensively described in literature like: Van Olphen, H., xe2x80x9cAn Introduction to Clay Colloid Chemistryxe2x80x9d, second edition, John Wiley and Sons, New York, 19977, p. 30-; and Darley, H. C. H. and Gray, George R., xe2x80x9cCompositions and Properties of Drilling and Completion Fluidsxe2x80x9d 5th edition, Gulf Publishing Company, Houston, p. 140-.
Some cellulose derivatives are known additives in drilling fluids. They may posses shale inhibiting-, viscosifying-, fluid loss reducing- and thermo stabilizing properties. Especially carboxy methyl cellulose (CMC) has been used as such an additive.
The modified cellulose product in accordance with the invention comprises cellulose ether, such as carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose or carboxy methylated starches, at least one ligand selected from the group consisting of nitrilo-tri-acetic acid, ethylene-di-amine-tetra-acetic acid, 1,2-cyclo-hexan-di-amine-N,N,Nxe2x80x2,Nxe2x80x2-tetra-acetic acid, di-ethylene-tri-amine-penta-acetic acid, ethylene-di-oxy-bis (ethylene-nitrilo)-tetra-acetic acid, (N-(2-hydroxy-ethyl)-ethylene-diamine-N,Nxe2x80x2,Nxe2x80x2-tri-acetic acid, tri-ethylene-tetra-amine-hexa-acetic acid, N-(hydroxyethyl)ethylene-di-amine-tri-acetic acid, and at least one salt of calcium.
The product thus comprises a calcium source [e.g. CaCl2.2H2O or Ca(OH)2]. The product can be used especially as an additive in drilling fluids. The application of this product leads to a more stable shale-formation without the detrimental effects of the hydration. Calcium ions are environmentally not harmful either in the preparation of the product or in its use. A further advantage in the use is that calcium is not amphoteric.
The products according to the invention can also be used in other applications than drilling fluids. Examples of such use are paper production, coating colors, mining, pharmaceuticals, water purification, ceramics, detergents, and paints.
Another object of the invention is a method for the preparation of the modified cellulose product, the method being characterized in that a cellulose compound is reacted with at least one salt of calcium and with at least one said ligand.
A third object of the invention is a drilling fluid containing the new product.
A fourth object of the invention is the inhibition of shale hydration during drilling by using the new product during drilling.
A fifth independent object of the invention is a product consisting of cellulose ether and said ligand. The ligand may be in the form of a monovalent salt, such as sodium salt. When there is no divalent cation present, the ligand forms direct linkage to the cellulose ether molecules.
The cellulose ether used in the invention may be e.g. carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose or carboxy methylated starches. Especially carboxymethyl cellulose can be used.
It is believed that in the new product calcium ions link the ether group of the cellulose molecule and the ligand(s).
The degree of polymerisation of the cellulose ether influences the viscosity of the product. The higher the polymerisation degree, the higher the viscosity of the resulting product. Suitable degree of polymerisation is e.g. 200-4000.
The degree of substitution of the ether is e.g. 0.5-1.7, especially 0.9-1.5.
The amount of salt used is especially in the range of 0.1-20 wt-%, preferably 0.1-15 wt-%, and most preferably 0.1-5 wt-%, based on dry cellulose. The amount of ligand used is especially in the range of 0.1-20 wt-%, preferably 0.1-5 wt-%, based on dry cellulose.
According to a preferred embodiment of the invention, the ligand is nitrilotriacetic acid.
The product according to the invention can be obtained in a technical form or in a purified form. In technical form it may contain e.g. from 1 to 45 wt-% of sodium chloride and/or sodium glycolates. In purified form it may contain the active ingredient even in a content between 95 and 100 wt-%, such as over 98 wt-%.
The product may comprise also a least one Mg2+, Cr3+, Fe2+, Fe 3+, Al3+, Ti4+, Zn2+, Sn2+, Be2+, Sr2+, Ba2+ or Ra2+ salt.
The fact that the compound can be used as a so-called xe2x80x9cone-bag productxe2x80x9d makes it very versatile in its application processes.
The product can be prepared by reacting cellulose or its derivative with the calcium salt and the ligand. This can be done e.g. by reacting the cellulose with a calcium salt and ligand at an elevated temperature of 40-80xc2x0 C.
The drilling fluid in accordance with the invention is easy to prepare by mixing the product with other necessary additives. The concentration of the modified cellulose product is e.g. 0.1-15 kg/m3. The make-up water of the drilling fluid may be fresh or distilled water, seawater or salt saturated water or mixtures thereof.
The shale inhibition performance of a modified CMC product according to the invention in a so-called dispersion test proved that this product was more effective as conventional CMC.
The drilling fluid according to the invention may also contain a glycol and/or potassium chloride (KCl) or any other salt, such as Mg2+, Cr3+, Fe2+, Fe3+, Al3+, Ti4+, Zn2+, Sn2+, Be2+, Sr2+, Ba2+ or Ra2+ salts.
Manufacturing
There is a lot of literature on the manufacture of cellulose ethers. Cellulose ethers are generally prepared by mixing wood-based or cotton wool-based raw material with a reaction medium, such as an alcohol (e.g. isopropanol or ethanol) or acetone, and by mercerising it with an alkali substance, such as sodium hydroxide, to activate the cellulose. An etherifying compound e.g. monochloroacetic acid (MCA) in liquid form or its sodium salt (NaMCA) is then added and allowed to react. The resulting product can be neutralized and its viscosity can be reduced if needed. When purified cellulose ether is manufactured, the by-products like salts generated in reaction are washed out with alcohol. The by-products can also be left in the product (technical cellulose ether) or removed only partly (semi-purified cellulose ether). The solvent used is separated and the product is dried. The product can be grinded, granulated or sieved, according to the intended end-use.
The degree of substitution for the e.g. carboxy methyl cellulose part of the product is determined by the reaction kinetics of the process and the concentration of monochloroacetic acid or its sodium salt and the type of cellulose used.
The calcium compound according to the invention and the complexing agent can be added at any point of the reaction before drying the product but the addition at the beginning of the reaction after feeding of the etherifying compound is preferred.